In the related art, in order to grasp the shape, structure, and the like of a biological sample such as a blood vessel or a cell, an imaging apparatus and an imaging method using an optical method have been developed (refer to Patent Literature 1).
In an imaging system disclosed in Patent Literature 1, an interference light image by light obtained by reflection of light from a light emitting unit on an object and interference is captured at a first timing, and a luminescent image of the light emitted from the object is captured at a second timing.
On the other hand, in an imaging technique using an optical method in the case of using a flow path such as a blood vessel as an imaging object, there is a concern that occurrence of various noises may cause detection accuracy to deteriorate. As one of the noises, speckle is known. The speckle is a phenomenon in which a spot-like swaying pattern appears on an irradiated surface depending on an uneven shape of the irradiated surface. In recent years, techniques have also been developed with respect to a method of imaging a flow path such as a blood vessel by using speckle which is one of the noises.
Speckle is a random interference/diffraction pattern due to scattering or the like in an optical path. In addition, the magnitude of speckle is represented by an index called speckle contrast which is a value obtained by dividing the standard deviation of the intensity distribution by the average of the intensity distribution. When the imaging object irradiated with coherent light is observed by using an imaging optical system, the speckle caused by scattering of the imaging object is observed on the image plane. Then, when the imaging object moves or changes in shape, a random speckle pattern corresponding to the movement or change is observed.
When a light scattering fluid such as blood is observed, the speckle pattern changes at every moment according to the change in fine shape caused by the flow. At that time, when an imaging element is arranged on the image plane and the fluid is imaged with an exposure time sufficiently longer than the change of the speckle pattern, the speckle contrast of a portion in which the blood is flowing, that is, a portion of the blood vessel is reduced in time average. Angiography can be performed by using such a change in speckle contrast.